Successful development of a fiber optic pH probe has provided a model for the extension of this approach to the construction of other probes, the most important being an oxygen probe. Effort during the previous year had been devoted to attempting to develop a suitable color absorption indicator for oxygen, a continuation of this effort led to the conclusion that this was too difficult a problem to solve, so the work was redirected toward evaluating the feasibility of using the principle of oxygen quenching of fluorescence.